1. Field of the Invention
The present invention relates, generally, to a method of fabricating a rigid-flexible printed circuit board (rigid-flexible PCB), and more particularly, to a method of fabricating a flying tail type rigid-flexible PCB that eliminates the need for a resist cover serving to protect a pad portion exposed for use in an external pad and a mounting pad from external environments, by window etching the base copper foil of a flexible region upon the formation of an external circuit pattern, instead of using the resist cover.
2. Description of the Related Art
Recently, while the degree of integration of semiconductor devices is gradually increasing, the number of pads provided on semiconductor devices to connect the semiconductor devices to external circuits is increasing and the mounting density is also increasing. For example, when a minimum processing dimension on the semiconductor device formed of silicon is about 0.2 μm, about 1000 pads may be provided on a semiconductor device having a size of about 10×10 mm.
Further, in semiconductor apparatuses, such as semiconductor packages, having the semiconductor devices mounted thereon, the size and thickness of the apparatus should be decreased to increase the mounting density. In particular, small and thin semiconductor packages are required for portable information apparatuses, such as notebook type PCs, PDAs, mobile phones, etc.
For packaging the semiconductor device, while the semiconductor device is mounted on a wiring substrate, the pad of the semiconductor device should be connected to the pad of the wiring substrate. However, in the case where about 1000 pads are provided on the about 10×10 mm sized semiconductor device, they come to have fine pitches of about 40 μm. Hence, with the aim of connecting the pad having the fine pitch of the semiconductor device to the pad of the wiring substrate, very high accuracy is required for wiring on the wiring substrate or position-matching upon connection. Eventually, it is difficult to apply conventional wire bonding techniques or tape automated bonding (TAB) techniques.
To solve the problem, the use of a rigid-flexible PCB having rigid and flexible regions interconnected without the use of an additional connector by structurally bonding a rigid substrate and a flexible substrate together is more and more frequently proposed. In particular, the rigid-flexible PCB is mainly applied to small terminals, such as mobile phones, realizing high integration by removing unnecessary space due to the use of the connector, depending on requirements of fine pitches and high integration of mounting parts in proportion to high functionality of the mobile apparatuses.
Although the rigid-flexible substrate is manufactured in the most commonly used rigid-flexible-rigid form or the rigid-flexible form, the present invention is directed to a flying tail type comprising only rigid-flexible regions.
Referring to FIGS. 1A to 1I, a conventional process of fabricating a flying tail type rigid-flexible PCB is sequentially shown.
In a polyimide copper clad laminate 10 including a polyimide layer 11 and a copper foil 12, the copper foil 12 is subjected to a photolithographic process to form an internal circuit pattern having a predetermined shape (FIG. 1A).
Then, to protect the internal circuit pattern corresponding to a flexible region, which is to be formed on the polyimide copper clad laminate 10, from the external environment, a polyimide film 20 is processed to suit the flexible region.
The processed polyimide film 20 is attached to the part of the flexible region having the corresponding internal circuit pattern, using an adhesive, after which the polyimide film 20 is temporarily bonded through manual soldering, thereby completing the process of forming a coverlay (FIG. 1B).
After the formation of the coverlay corresponding to the part of the flexible region using the polyimide film 20, a resist cover 30 is formed on the other part of the flexible region (FIG. 1C).
The resist cover 30 functions to protect the internal circuit pattern exposed for use in an external pad and a mounting pad from external environments, and includes, for example, heat-resistant tape or peelable ink. In addition, the resist cover 30 is removed after the completion of the substrate, unlike the polyimide film 20.
Subsequently, to confer mechanical strength and adhesive strength to a rigid region, which is to be formed on the polyimide copper clad laminate 10, prepregs 40 and base copper plates 50 are laminated on the upper and lower surfaces of the base substrate to face each other.
After the prepregs 40 and the base copper plates 50 are laminated, they are compressed using a press, to form both a rigid region, in which the circuit pattern is incorporated into the prepreg, and a flexible region, in which the circuit pattern is covered with the coverlay (FIG. 1D).
Then, a through hole 60 for electrical connection between internal and external layers is formed (FIG. 1E). The base copper plate 50 and the through hole 60 are plated with copper to form a copper plated layer 70 (FIG. 1F). During the plating, an external circuit pattern having a predetermined shape is formed (FIG. 1G).
The external circuit pattern is obtained using a predetermined photolithographic process, and the plated layer of the flexible region is etched therewith.
After the external circuit pattern having a predetermined shape is formed, the resist cover 30 is removed.
In the case where the resist cover is formed of peelable ink, peelable ink may be easily removed by further applying peelable ink on the formed peelable ink to form an ink layer having a predetermined thickness and then removing the ink layer (FIG. 1H).
Thereafter, the resultant substrate is coated with PSR ink (photo imageable solder resist mask ink) and then surface treated, thereby finally completing a flying tail type rigid-flexible PCB, in which the flexible region is covered with the coverlay, including the polyimide film and the resist cover (FIG. 1I).
However, in the flying tail type rigid-flexible PCB thus formed, since the external pad or mounting pad of the flexible region is internally exposed, processes of forming the resist cover on the flexible region using the heat-resistant tape or peelable ink and then removing it are required, causing problems of undesirably complicating fabrication processes and of increasing fabrication costs.
Further, due to the residue remaining after the removal of the heat-resistant tape or peelable ink, contamination may occur, thus a defect rate is increased, resulting in drastically decreased reliability.